Method of prestressing concrete pressure tanks

ABSTRACT

A method of preparing parallel-wire cable for use in prestressed concrete pressure tanks comprises the steps of cutting the cable in lengths with each wire of equal length, twisting the lengths of cable at least twice about the longitudinal axis of the cable, and preferably from two to five times, and positioning the lengths of cable within the tank wall prior to stressing the cable. The cable is preferably twisted before it is placed in position in the tank. However, it may be twisted after insertion into passageways within the concrete wall of the tank.

United States Patent 1 Birkenmaier et a1.

[54] METHOD OF PRESTRESSING CONCRETE PRESSURE TANKS [76] Inventors: Max Birkenmaier; Antonio Brandestini, both of Sophienstrasse 16, Zurich, Switzerland; Mirko Robin Ros, deceased, late of Zurich, Switzerland; Erika Emma Ros nee Marti, Schwendenhausstrasse, 8702 Z01- likon Z11, Switzerland; Franziska Jelena Saager nee Ros, Schwendenhausstrasse, 8702 Zollikon Zl-l, Switzerland; Cornelia Erica Ros, Schwendenhausstrasse 9, 8702 Z01- likon ZH, Switzerland heirs [22] Filed: Aug. 12, 1970 [21] Appl. No.: 63,170

Related US. Application Data [63] Continuation-in-part of Ser. No. 667,402, Sept. 13,

1967, abandoned.

[30] Foreign Application Priority Data Sept. 14, 1966 Switzerland ..G 13369/66 [51] Int. Cl. ..B21d 39/00 [58] Field of Search ..29/505, 433, 470.5,

[ 1] 3,725,999 [451 Apr. 10, 1973 [56] References Cited UNITED STATES PATENTS 3,086,273 4/1963 Welborn......,..... ....52/223 R 3,187,466 6/1965 Zerr ....52/223 R 3,526,034 9/1970 Beurrier... ..29/505 3,616,531 11/1971 Tyler l ..29/505 2,216,758 10/1940 Schmidt ..29/505 3,059,366 10/1962 Kafka ..29/505 3,131,469 5/1964 Primary Examiner-Richard J. Herbst Assistant Examiner-D. C. Crane Attorney--McGlew and Toren [57] ABSTRACT A method of preparing parallel-wire cable for use in prestressed concrete pressure tanks comprises the steps of cutting the cable in lengths with each wire of equal length, twisting the lengths of cable at least twice about the longitudinal axis of the cable, and preferably from two to five times, and positioning the lengths of cable within the tank wall prior to stressing the cable. The cable is preferably twisted before it is placed in position in the tank. However, it may be twisted after insertion into passageways within the concrete wall of the tank.

5 Claims, 9 Drawing Figures Glaze ..29/505 PA'rErminMR 1 01m 9 725 999 SHEET 1 BF 2 Flt-1.20

I F l G. 40

C INVENTORS MAX BIRKENMAIER ANTONIO BRANDESTINI MlRKO ROBIN ROS F |G.3 by

ATTORNEYS PATENTEU 1 1 5 SHEET 2 BF 2 ATTORNEYS METHOD OF PRESTRESSING CONCRETE PRESSURE TANKS This is a continuation-in-part of our copending application Ser. No. 667,402 filed Sept. 13, 1967, now abandoned.

BACKGROUND'OF THE INVENTION SUMMARY OF THE INVENTION The present invention is directed to a method of preparing cables for use in prestressing concrete pressure tanks and, more particularly, it is concerned with a method for preparing parallel-wire cables for use in prestressing such concrete tanks.

High pressure tanks made of prestressed concrete are used in many different fields at the present time, and particularly in atomic power plants. In atomic plants, the reactor and the heat exchanger are located in a large, generally cylindrical, chamber which is under a gas pressure of 30 or more atmospheres. In order to withstand the high internal pressures developed in such concrete pressure tanks, cables are positioned within the concrete walls of the tank and are prestressed to 600 tons or more. In accordance with the present practice, the cables are placed in tubes or passageways extending through the concrete wall and then are prestressed in place.

Certain economical and technical advantages are achieved if parallel-wire cable is employed in prestressing the concrete. However, certain problems also follow from the use of such cable. As an example, a parallel-wire cable consists of 180 wireseach having a diameter of 7mm., which form a wire bundle having a diameter of 7 to 8 cm. Since the looping angle of the cable when positioned for prestressing concrete usually amounts to between 180 to 270 and more, considerable differences in length occur between the inner and outer wires of the cable. The difference in length under certain conditions can amount to between to 30 cm. Such differences in length are undesirable because, during stressing, the wires are unevenly loaded and this usually leads to a premature failure of the shorter wires. A non-uniform cutting of the individual wires in the cable corresponding to the different positions they as sume when the cable is arranged in a curved position is impractical, and when stressing the cables, it is difficult to secure all of the wires in the desired position. One possibility which has been considered has been to cut the individual wires of the cable after the cable has been stressed, that is, in a curved state. However, such a procedure makes it impossible to fabricate the cables away from the construction site and, as a result, use of such cables is undesirable for reasons of both time and of cost.

Therefore, it is the primary object of the present invention to utilize parallel-wire cables, which have been twisted a limited number of times about their longitudinal axes, in prestressing concrete pressure tanks, for avoiding the problems previously experienced with the use of such cable.

Another object of the present invention is to employ a parallel-wire cable which has been twisted in a limited manner so that the difference in length of the individual wires within the cable in its looped or curved orientation in the concrete pressure tank is limited.

Still another object of the present invention is to provide a twisted parallel-wire cable which can be fabricated in its twisted form away from the construction site of the tank and can be used in prestressing the tank with a minimum of difficulty.

Moreover, another object of the invention is to provide lengths of parallel-wire cable twisted through a limited number of turns for use in prestressing concrete pressure tanks, which cable lengths are simple to form, easy to handle and are relatively inexpensive in cost.

Accordingly, in the present invention a length of parallel-wire cable is cut so that the individual wires are of equal lengths and then the length of cable is twisted at least twice about its longitudinal axis over its entire length before it is used in prestressing a concrete tank. The differences between a conventional twisted wire cable and a parallel-wire cable twisted only a limited number of turns over its entire length can be readily ap- 'preciated.

Because of the twisted orientation of the individual wires forming the cable, the difference in length between diametrically opposed inner and outer wires is very considerably limited when the cable is in a curved or looped position. It has been shown that by rotating or twisting the cable through a selected number of revolutions about its longitudinal axis it is possible to reduce the differences in length by one-tenth as compared to that which exists when a length of parallelwire cable is employed in the untwisted state. This limitation in the lengths of individual diametrically opposed inner and outer wires is sufficient to insure comparative uniformity in the tensile strength of all the wires forming the cable.

In employing the present invention, it is advantageous both to cut the parallel-wire cable and to rotate or twist it at its place of manufacture rather than at the construction site of the tank which is to be prestressed. However, it is also possible to bring the individual cut lengths of cable to the construction site and to twist them only after the cable has been inserted in the passageways in the tank walls. In either case, however, it is necessary that the lengths of cable be twisted before they are stressed.

The various features of novelty which characterize the invention are pointed out with particularity in the claims-annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a schematic showing of a length of parallelwire cable, twisted about its longitudinal axis and arranged in a looping angle of 270 with the position of the wires in the cable rotated through 180 for the length of the angle of 270, and with a wire shown in the cable having its bisecting point located at the inner edge of the cable;

FIGS. 2a, 2b and 2c each shows a cross sectional view through the cable taken along the lines AA, B- B and CC of FIG. 1, respectively;

FIG. 3 is a schematic showing similar to that in FIG. 1 of a cable wound into a looping angle of 270, however, with a wire shown in the cable having its bisecting point located at the outer edge of the cable;

FIGS. 4a, 4b and 4c each show a cross sectional view through the cable taken, along lines A-A, BB and CC of FIG. 3, respectively; and

FIG. 5 is a graph representing the differences in length of individual diametrically opposed inner and outer wires in a parallel-wire cable, when disposed in a curved orientation, based on the number of complete revolutions or twists made in the cable about its longitudinal axis.

DETAILED DESCRIPTION OF THE INVENTION In a preferred embodiment of the method, according to the present invention, a parallel-wire cable, that is a cable formed of a plurality of wires all arranged in parallel relationship as compared to a conventional twisted wire cable, is cut in the shop into a number of pieces with the individual wires in each piece being of equal length and each piece or length provided at one end with a prestressing stay. Each individual piece of cable is twisted or rotated two to five times about its longitudinal axis over the full length of the piece. When the twisting of the individual lengths of the cable is completed, the wire bundle forming each length of the cable is bound or banded about its circumferential periphery at regular intervals along its length to assure the relative position of the individual wires in the cable.

The banding is for ease in handling and transporting the twisted parallel-wire cable and is not a requisite of the invention. Due to the slight twisting action performed on the individual pieces of parallel-wire cable over their lengths there is no problem in the cable becoming untwisted. After winding the individual lengths of cable on a drum, they can be brought to a construction site for placement within the walls of the concrete tank.

Prior to inserting and prestressing the lengths of cable, the concrete tank is constructed in place, and separate sheet metal tubes are provided through its walls running in the circumferential direction to receive the individual lengths of cable. Next, the individual lengths or sections of cable are withdrawn from the drum and are inserted into the tubes in the wall. The free end of each length of cable, that is, the end which does not have a prestressing stay, is inserted first into the tubes. The banding means which are placed about the sections of cable after it has been twisted are removed before the wire is inserted into the wall. With the banding means removed, any friction losses that might be developed during stressing are reduced and any additional strains which might have been developed are avoided. The individual lengths of cable remain in the twisted condition as they are inserted and since the extent to which the individual lengths have been twisted is slight, there is no tendency for the lengths to become untwisted. After each length of cable is fully inserted through the tube a stay is attached to its free end. Finally, the individual cable sections are stressed and the stays are fixed in a stressed position.

The details of the concrete tank and its metal tubes,of the prestressing stays, and of the prestressing operation do not form a part of the invention and, since they are conventional, further description and illustration is not considered, necessary.

It will be appreciated that as the number of turns or revolutions given a length of cable are increased,the difference in length between the diametrically opposed inner and outer wires keeps decreasing when the length of cable is in a curved disposition. However, in this procedure, the difference in length between an outer wire and a wire located at the center of the bundle of wires in the cable becomes greater. It has been found that the best result is obtained if the twisting action is maintained within a relatively narrow range, that is, if it is limited to between two and five rotations. For reasons of safety, an average number of three to four rotations has been considered most advantageous.

In the graph illustrated in FIG. 5, the vertical axis indicates the difference in length between an inner wire and an outer wire, or an outer and a centrally disposed wire, of a curved length of parallel-wire cable while the horizontal axis indicates the number of twists or revolutions given to the length of parallel-wire cable about its axis. The curve A indicates the difference in length between two diametrically opposed inner and outer wires in a curved length of cable based on the amount of rotation imparted to the cable. The multiple curves B show the difference in length for different average curve radii of the cable based on an outer wire and a centrally positioned wire in the cable. From the lefthand end of curve A it can be readily appreciated that between zero and one complete revolution or twist of a length of cable, a considerable difference in length occurs between diametrically opposed inner and outer wires in the cable. If a small error is made in the amount of rotation given to the cable, a very considerable difference in length will occur. Further, it can be seen in FIG. 5 that the differences in length between two diametrically opposed wires drop very considerably after two complete rotations of the cable have been effected, and the differences in length level off beyond three complete rotations. On the other hand, the difference in length between an outer wire and a centrally disposed wire increases progressively with the number of turns given the cable.

Accordingly, the usable range of the twisted lengths of parallel-wire cable, for the various cases under consideration, is beyond two complete rotations of the cable length and is dependent upon the radius in which the cable is placed. However, the number of twists which are imparted to an individual length of cable is determined not only by the difference in length between two diametrically opposed wires in the cable bundle but also by the difference in length between an outer wire and a centrally disposed one. The optimum condition is where the difference in length between two diametrically opposed outer wires is at a minimum and the comparable difference in length between a centrally disposed wire and one of the outer wires is also at a minimum. Therefore, the preferable range exists between two complete revolutions (411) and five complete revolutions (l01r) as shown in FIG. 5.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. A method of preparing individual lengths of parallel-wire cable, of the type comprising a plurality of parallel wires, including a central wire, arranged in a substantially circular cross-section bundle, for use individually as respective cable lengths for insertion in curved walls of concrete pressure tanks for prestressing such curved walls, said method comprising the steps of cutting such a parallel wire cable into such individual lengths in which all the wires have the same length; and

separately imparting to each individual length, between its ends, not less than two and not more than five'twists in the same direction about its longitudinal axis and then stressing each length as arranged to extend 1 1 5. A method, as set forth in claim 1, comprising twistthrough a respective looping angle within the curved tank wall.

ing the individual lengths of parallel-wire cable four times about the longitudinal axis thereof. 

1. A method of preparing individual lengths of parallel-wire cable, of the type comprising a plurality of parallel wires, including a central wire, arranged in a substantially circular cross-section bundle, for use individually as respective cable lengths for insertion in curved walls of concrete pressure tanks for prestressing such curved walls, said method comprising the steps of cutting such a parallel wire cable into such individual lengths in which all the wires have the same length; and separately imparting to each individual length, between its ends, not less than two and not more than five twists in the same direction about its longitudinal axis and then stressing each length as arranged to extend through a respective looping angle within the curved tank wall.
 2. A method, as set forth in claim 1, comprising twisting the individual lengths of parallel-wire cable before inserting the lengths into the tank wall.
 3. A method, as set forth in claim 1, comprising inserting the individual lengths of parallel-wire cable through looping angles into the tank wall and twisting the individual lengths after they are positioned in the wall.
 4. A method, as set forth in claim 1, comprising twisting the individual lengths of parallel-wire cable three times about the longitudinal axis thereof.
 5. A method, as set forth in claim 1, comprising twisting the individual lengths of parallel-wire cable four times about the longitudinal axis thereof. 